WC and brute are not recommended for this boss, they both have low magic resist and this boss has high magic damage
Use CC units instead of tanks,pyscho as a lone tank will be enough as long as you don't let boss fire off ultis. If he fires a ulti off, you lose so make sure to stop it with pyscho/silencer US
Good CCs are warrior monk/silencer/ ice mage/ chap/ shadowleaf.
No need to wait for 85, I completed stage 3 at 81

thx for the info, brute magic resis is average, i put him cause overtime . still testing, other bosses is fine in lv3, only this fighter.... thx again.

I myself haven passed it, the difficulty of the stage is mostly just having your frontline tanks have enough stars/promotions/level to pass.
I'll say the best units are probably
DK(to tank+heal/shield, also aura to reduce damages from boss magic spear)
Panda(To blind boss but make sure not to use his US, will cause him to move forward, making boss attack only him)
Mystic/Machinist( also to blind)
Chaplain(to heal)
Disease Bringer(VIP only,to summon extra tank)
Monkey(VIP only,Clones to divert fire from your tanks)

Uploaded this picture for us more visual people. OP you are more than welcome to use this in your first post. Don't need to credit me. I found it on the HC Vietnam Club Facebook page.

Thanks for the picture but some of the info is not helpful so I don't think I'll add to the first post.
For example, WM against phoenix is a bad idea, that boss is magic immune so most of WMs attack don't work,letting the boss US with a higher HP, likely killing more of your units than it should.